PROTECT: Puerto Rico Testsite for Exploring Contamination Threats

1. PROTECT: Puerto Rico Testsite for Exploring Contamination Threats Modeling NAPL Transport in the North Coast Karst Aquifer, Puerto Rico C. Butscher, R. Ghasemizadeh, I. Padilla, A. Alshawabkeh Challenges in karst. Karst systems have a high degree of heterogeneity (background picture and small photo), which makes them behave very differently from other aquifers. Slow seepage through the rock matrix and fast flow through conduits (Fig. 1) result in a high variation in aquifer behavior. Contaminant storage occurs in the rock matrix, but contaminant transport occurs mostly along preferential pathways in the conduit network (Ghasemizadeh et al. in prep.). Computer models predicting contaminant transport must be able to simulate combined matrix and conduit flow (Fig. 2). The properties of the conduit network, however, are difficult to determine. Why modeling? The prediction of water resources and the assessment of contaminant pathways toward exposure demand quantitative simulation tools. Groundwater models can be a major help in determining potential paths for exposure of superfund related contaminants in the North Coast Limestone aquifer of Puerto Rico. With this knowledge, strategies can be undertaken to reduce exposure and protect public health. NAPL pool Fig. 2:“Bath tub” model symbolizing mixed reactor models to represent karst groundwater systems(from Butscherand Huggenberger, 2009). Fig. 1: Conceptual model of a karst aquifer (Butscher and Huggenberger, 2009). Dynamic transport of NAPLs. Non-aqueous phase liquids (NAPLs) are important superfund-related contaminants. They behave differently than water in the subsurface: depending on their density, they float and accumulate on the water table, or tend to sink vertically to the aquifer basis. The effect of different flow regimes on NAPL transport is poorly understood in karst systems (White 2002). Pooled NAPLs remain stationary under base flow conditions. During storm flows, NAPL pools can be dragged downstream or flushed as suspensions (Vesper et al. 2001). As a result, storm flow can send previously immobile NAPLs to outflows in toxic pulses. A need exits to advance knowledge on the main processes controlling NAPL fate and transport in karst groundwater systems characterized by variable conduit and diffuse flow. Fig. 4: Distributed finite difference model from the Manatí – Vega Baja area, Puerto Rico (Cherry, 2001) • Suggested approach: • Distributedregional scale model of central part of North Coast Aquifer, PR (Fig. 3). Covers large area. Data collected by Project 4 and documented in scientific reports (Cherry 2001). • Modelsat a smaller scale in Vega Alta region (Fig. 3). Extensive monitoring data available. • Spatially lumpedmodel (“mixed reactors”, Fig. 2). Applicable where conduit system cannot be spatially resolved (Butscher and Huggenberger 2009). Simulation of water and NAPL phase in one model. • Spatially distributed model (c.f., Fig.4). Finite difference model MODFLOW-2005-CFP (Hill et al. 2010). Couples continuum (rock matrix) with discrete conduit network to simulate combined conduit and diffuse flow. Interpretive character because of unknown geometry of conduit network. Links contamination with spatially distributed flow conditions. Combination with lumped model allows minimizing interpretive character. This program is supported by Award Number P42ES017198 from the National Institute of Environmental Health Sciences. References: Butscher C, Huggenberger P, 2009. Modeling the Temporal Variability of Karst Groundwater Vulnerability, with Implications for Climate Change. Environ. Sci. Technol. 43, 1665-1669. Cherry GS, 2001. Simulation of flow in the upper north coast limestone aquifer, Manatí-Vega Baja area, Puerto Rico. USGS Water-Resources Investigations Report 00-4266, 82 p. Ghasemizadeh R, Hellweger F, Butscher C, Padilla I, Vesper D, Field M, Alshawabkeh A, in prep. Review: Groundwater flow and transport modeling of karst aquifers, with particular reference to the North Coast Aquifer of Puerto Rico. To be submitted to Hydrogeol. J. Hill, ME, Stewart, MT, Martin, A, 2010. Evaluation of the MODFLOW-2005 Conduit Flow Process. Ground Water 48 (4), 549-559. Vesper, DJ, Loop, CM, White, WB, 2001. Contaminant Transport in Karst Aquifers. Theoretical and Applied Karstology 13-14, 101-111. White, WB, 2002. Karst hydrology: Recent developments and open questions. Eng. Geol. 65 (2–3), 85–105. Fig. 3: Suggested model areas in the North Coast Aquifer of Puerto Rico (map from Cherry 2001). } www.neu.edu/protect